Heretofore, work on optical fiber amplifiers has been concentrated on single mode fibers. An example of a design for an optical fiber amplifier using a single mode fiber is contained in the disclosure of U.S. Pat. No. 5,005,175, Desurvire et al, issued Apr. 2, 1991. This disclosure to the extent relevant is hereby incorporated by reference.
Extending the use of optical fiber amplification to multi-mode fibers is attractive because of the use of multi-mode fibers in applications such as local area networks, switching systems, and fiber sensors.
In optical fiber amplifier designs for single mode fibers, optical noise generally is not so large so as to limit the optical amplifier's usefulness. Noise is proportional to the number of propagating modes, as well as the degree of population inversion along the amplifier length. In such applications, noise generated in the amplification process is limited by the fact that only two orthogonally polarized modes are propagating in the fiber amplifier. In adapting multi-mode fibers to optical fiber amplifiers, however, noise generation incident to the amplification presents a complex set of issues.
The amount of noise generated is related to the number of modes that can be guided in the multi-mode fiber. Signal amplification, however, is dependent on the overlap between signal modes and excitation pump modes within the fiber regions containing concentrations of dopant such as erbium. For the case of a uniformly doped multi-mode fiber core, as the number of pump modes increases, signal gain increases; however, a point is rapidly reached at which the number of excited noise modes is high enough to overwhelm the multi-mode signal with noise. It follows, therefore, that if all or even most of the hundreds of modes of a multi-mode fiber amplifier are exited by the pump, the resulting noise factor is too high to allow optical amplification for any practical system application.